High intensity discharge (HID) lamps are typically used where large areas require illumination, such as in factories, parking lots and sports arenas. In some applications, such as illuminating a sports arena during a sporting event, after a momentary power failure that terminates illumination by the lamps, it is naturally desired that the lamps rapidly restart to allow the sporting event to continue. However, a hot HID lamp typically requires a high current at an elevated voltage to cause the lamp to drop in voltage to where its power supply, or ballast, circuit can sustain lamp operation.
The above cross-referenced application entitled "System for Starting a High Intensity Discharge Lamp," Ser. No. (applicant docket no. LD-10519), is directed to a hot restrike circuit for starting extinguished HID (e.g. metal halide) lamps that are still hot. With such hot restrike circuit, multiple high voltage pulses per half cycle of lamp-driving voltage are provided to assure the high current at an elevated voltage needed to initiate glow-to-arc transition, i.e. , lamp turn-on. Such high voltage pulses are referred to herein as "hot restrike" pulses. The above cross-referenced application entitled "Regulation of Hot Restrike Pulse Intensity and Repetition," Ser. No. (applicant docket no. LD 10738), is directed to further improvements in a hot restrike circuit. One improvement is to regulate the intensity of hot restrike pulses so that they are consistently at a high level. As such, the hot restrike pulses are more effective at delivering the high current to the lamp needed to achieve glow-to-arc transition. Another improvement is to increase the reliability of obtaining multiple hot restrike pulses during a half cycle of lamp-driving voltage, which also contributes to the effectiveness of the hot restrike pulses.
Over the life of a lamp, however, hot restrike capability is prone to degrading. Degrading also typically occurs when a hot restrike circuit is operated in higher temperature ambients, or when the voltage of a power source decreases. Such degrading of hot restrike performance can become so severe that a lamp will fail to achieve glow-to-arc transition, i.e., fail to start. One approach to remedying such degradation would be to modify the hot restrike circuitry to increase the pulse rate and energy in each hot restrike pulse. This approach, however, normally involves using significantly larger components in the hot restrike circuitry (e.g. capacitors), or adding more winding turns to a ballast transformer, which often are impractical. It would, therefore, be desirable to enhance the effectiveness of hot restrike circuitry so that it reliably works even in such extreme conditions, and also without any substantial increase in size of the hot restrike circuitry or the ballast transformer.